The in-situ delivery of therapeutic agents within the body of a patient is common in the practice of modern medicine. In-situ delivery of therapeutic agents is often implemented using medical devices that may be temporarily or permanently placed at a target site within the body. These medical devices can be maintained, as required, at their target sites for short or prolonged periods of time, in order to deliver therapeutic agents to the target site.
Nanoporous materials have the potential to revolutionize drug delivery. For example,
iMEDD, Inc. has created silicon membranes with parallel channels ranging from 4 to 50 nm. Diffusion rates of various solutes through such membranes have been measured and conform to zero-order kinetics in some instances (i.e., release is constant with time). This is in contrast with typical situations in which drug diffusion rates decay with time, because the concentration gradient, and thus the driving force for diffusion, is also decaying with time. One explanation for zero order behavior is that, by making the diameter of the nanopores only slightly larger than that of the drug, the nanopores act as bottlenecks, forcing the drugs to proceed in a substantially single-file fashion through the membrane. iMedd claims that the membranes can be engineered to control rates of diffusion by adjusting channel width in relation to the size of solutes. When the proper balance is struck, zero-order diffusion kinetics is possible.
iMedd has produced a drug delivery device which consists of a drug-filled enclosure which is fitted with a nanoporous membrane as the only connection between the internal reservoir of the device and the external medium. These devices, however, do not have any function beyond drug delivery.
H. Wieneke, et al., “Synergistic effects of a novel nanoporous stent coating and tacrolimus on intima proliferation in rabbits,” Catheterization and Cardiovascular Interventions, Volume 60, Issue 3, pp. 399-407, describe stainless steel coronary stents that are provided with a ceramic nanoporous aluminum oxide (Al2O3) coating, which is used as a carrier for tacrolimus. Similarly, U.S. Patent Appln. Pub. No. 2005/0070989, describes implantable medical devices such as stents, which have nanoporous layers that are loaded with therapeutic agents. However, because the nanoporous layers in these devices also serve as the drug reservoirs, the amount of drug that may be loaded is limited.